Explanation:
Elliptical galaxy NGC 720 is enveloped in a
cosmic cloud of x-ray emitting gas.
Seen in
this
false color image from the
Chandra
X-ray Observatory,
the extreme temperature of the gas - about 7 million degrees Celsius -
makes it impossible to confine the cloud to the vicinity of NGC 720 based
on the gravity of this galaxy's visible stars alone.
In fact, the x-ray cloud is taken as solid evidence for the
presence
of dark matter surrounding NGC 720 -- unseen material which has
gravitational influence that can keep the x-ray hot gas cloud
from escaping.
Chandra's remarkable vision clearly distinguishes the bright
point-like x-ray
sources from the diffuse cloud.
Astronomers can then use
the detailed shape
of the cloud to infer
the distribution of dark matter in NGC 720 and even test theories
about the fundamental nature of dark matter.
According to modern understanding, the mysterious
dark matter, whatever
it is, is by far the most common
form of matter in the Universe.
Galaxy NGC 720 lies about 80 million light-years distant
toward the constellation Cetus.